Polyurethane foams with a wide variety of physical properties have long been produced on a technical scale by the isocyanate polyaddition process from compounds containing several active hydrogen atoms, and in particular compounds containing hydroxyl and/or carboxyl groups, polyisocyanates and water and/or organic blowing agents with the optional addition of catalysts, emulsifiers and other additives [Angew. Chem. A, 59 (1948), page 257]. With suitable choice of components, it is possible by this process to obtain either elastic or rigid foams or any variations between these extremes.
Polyurethane foams are preferably produced by mixing the liquid components. The starting materials which are to be reacted together are either all mixed together at once or an isocyanate prepolymer is first prepared from polyols and an excess of polyisocyanates, and this prepolymer is then foamed.
Tertiary amines have proved to be very suitable catalysts for the production of polyurethane foams, mainly because they accelerate both the reaction between the hydroxyl and/or carboxyl groups and the isocyanate groups and the reaction between water and isocyanate groups. In the one-shot process, the velocities of these reactions, which take place side by side, may be adjusted to each other.
Additional cross-linking reactions accompany the foaming process to form allophanate, biuret and cyanurate structures. In view of the complexity of the reactions, the catalysts must be suitably chosen to ensure that, on the one hand, the various reactions will be synchronized and that, on the other hand, the catalyst will not be fixed too early in the process by being incorporated in the foam and will not subsequently accelerate hydrolytic degradation of the finished foam. Furthermore, the unpleasant odor of many of the tertiary amines frequently used in practice is a disadvantage if they are to be used in the production of foams.
In U.S. Pat. No. 3,243,389, and German Offenlegungsschrift No. 2,354,952, tertiary amines which contain Zerewitinoffactive hydrogen atoms are described as catalysts for isocyanate polyaddition reactions. However, these compounds are gradually built into the polymer structure in the course of the foaming process. They are therefore no longer available at sufficiently high concentrations in the final stages of the reactions, especially in the marginal zone of the foams. This insufficiency adversely affects the surface characteristics and the gelling properties of the polyurethane foams.